Low drop out (LDO) regulators are devices that provide a nominal and stable DC voltage by adjusting their internal resistance to any occurring variation of a supplied load. Because of their functioning characteristics, LDO regulators may be embedded in power management ICs for collecting energy to adapt power interfaces between an energy storage device, such as a battery or a supercapacitor, and loads functioning with a low duty cycle. Microprocessors, analog sensors and RF transceivers are example loads functioning with a low duty cycle.
To meet these requirements, power management ICs maximize power transfer from an energy collecting source to a battery and to a supplied load, and reduce power consumption. Power consumption is reduced particularly during stand-by periods due to the low duty cycle of the supplied loads.
Furthermore, load currents may vary from values below 1 μA, in stand-by conditions, to several tens of mA during data processing and transmission. For this reason, another requirement for LDO regulators in energy collecting applications is a fast response to load variations with reduced undershoots and overshoots to avoid an unwanted reset of the supplied load (e.g., a microprocessor).
A well-known basic linear voltage regulator is depicted in the block scheme of FIG. 1a and in the corresponding circuit of FIG. 1b. The linear voltage regulator comprises an error amplifier ErrAmp configured to receive as an input a reference voltage Vref, typically generated by a band-gap circuit, and a feedback voltage Vfb. The feedback voltage Vfb represents the output voltage Vout_pch, and is configured to control a pass transistor Tpass, typically a PMOS enhancement FET. The pass transistor Tpass is biased in a conduction state to regulate the output voltage Vout_pch so as to make the feedback voltage Vfb match the reference voltage Vref. The amplifier may be a differential amplifier with an active load, as shown in FIG. 1b. The response to load variations of this basic regulator is relatively slow, and this may make it practically unsuitable for energy collecting applications.
Another known regulator is illustrated in the block scheme of FIG. 2a and in the corresponding circuit of FIG. 2b. In this regulator, the powering voltage Vboost of the error amplifier ErrAmp is a boosted replica of the supply voltage VDD. This causes the pass NMOS transistor Tpass to function in the linear functioning region of its current-voltage characteristic when the output regulated voltage Vout_nch is close to the supply voltage VDD. The boosted powering voltage Vboost is typically generated by a charge pump generator CHARGE PUMP controlled by an oscillator operating at a fixed frequency. The amplifier may be a differential amplifier with an active load, as shown in FIG. 2b, in which the voltages Vref and Vfb are applied to the input terminals of the amplifier to properly drive the NMOS pass transistor Tpass.
This regulator is characterized by a fast response to load variations due to the reduced size of the load supplying NMOS transistor. This is done at the cost of greater power consumption in inactive conditions due to the presence of a charge pump generator, which may make it unsuitable for energy collecting applications.
A low drop out (LDO) regulator capable of combining the contrasting requirements of a short transient response to load variations with a very small power consumption may be advantageous for realizing energy collecting devices with reduced power consumption, and thus with improved yield.